With the advent of stricter governmental controls for engine emissions and with increased concern being given to weight reduction of passenger vehicles to economize fuel, there is a need for conserving the residual heat of exhaust gases of an internal combustion engine so that downstream equipment in a vehicle exhaust system may operate with higher efficiency and effectiveness to reduce the emission levels of the engine. This need has become quite apparent to the automotive industry and is currently under intense development effort. Any solution to this problem must be simple yet durable and not introduce any additional problems. Heat loss experienced by the exhaust gases, as they travel from the combustion zone through the withdrawal or exhaust passages of the engine block to exterior exhaust tubing, can be considerable. Such heat loss is accomplished by conduction, convection and radiation. This heat loss can be particularly disadvantageous to devices, such as thermal reactors which depend heavily upon a high exhaust gas temperature to react additional air or oxygen for carrying on additional chemical inter-reaction promoting further emission control. Under knowledge of the prior art to date, the ability to maintain a sufficiently high temperature of the exhaust gas in an internal combustion engine has not been fully attainable.
Attempts by the prior art to suitably preserve the heat content of the exhaust gas, prior to entrance into or actual residence within a thermal reactor have consisted primarily of providing independent material type insulators (such as air gap assemblies, metal or nonmetal blankets) in the passages leading to the reactor to prevent loss of heat through the walls of the exhaust passages around the exhaust port, or in members surrounding the exhaust passages. The problems with a cast-in insulating layer is that it is very difficult to accommodate in production castings, and may tend to generate hot spots which are potentially troublesome since they may lead to cracking.